ICE-03 — The Otto and Diesel Cycles

ASSUMPTION — This lesson models the ideal (air-standard) cycles. The real cycle deviates due to heat losses, finite combustion duration and gas exchange. The aim is to grasp the thermodynamic basis of the two engine families.

In this lesson you will learn:

• The four basic stages of each cycle
• The heat-addition difference
• The effect of compression ratio on efficiency

1. Why Ideal Cycles?

Real combustion is complex. Engineers idealize combustion as “heat addition” to see the thermodynamic limits.

FACT — The Otto cycle models spark-ignition (gasoline) engines; the Diesel cycle models compression-ignition (diesel) engines. Both include: isentropic compression, heat addition, isentropic expansion, heat rejection.

2. Otto Cycle (Constant-Volume Heat Addition)

In a gasoline engine the mixture is ignited near the end of compression and burns very quickly — idealized as constant-volume heat addition.

FACT — In the Otto cycle, heat is added at constant volume near TDC, so pressure rises abruptly (a vertical line on the P-V diagram).

3. Diesel Cycle (Constant-Pressure Heat Addition)

In a diesel, fuel is injected into hot compressed air and burns as injection continues — idealized as constant-pressure heat addition.

FACT — In the Diesel cycle, heat is added at constant pressure (a horizontal line on the P-V diagram). Modern multiple-injection diesels complicate this picture (details: ICE-14).

4. Comparison

Feature	Otto	Diesel
Ignition	Spark	Compression heat
Heat addition	Constant volume	Constant pressure
Compression ratio	Low–medium (knock limit)	High
Efficiency trend	Medium	High

FACT — At the same compression ratio the Otto cycle has higher theoretical efficiency, but diesels can run much higher compression ratios (no knock) and thus reach higher real efficiency.

5. Compression Ratio and Efficiency

FACT — Ideal thermal efficiency rises with compression ratio. In gasoline engines it is limited by knock (ICE-06, ICE-27); diesel is not so limited.

6. Summary

• Otto = constant-volume, Diesel = constant-pressure heat addition.
• Both: compression → heat in → expansion → heat out.
• Diesel uses higher compression ratio (no knock limit).

7. Sources

• Heywood, Internal Combustion Engine Fundamentals
• Bosch Automotive Handbook

Next Lesson

• ICE-04 — Engine Geometry: bore, stroke, piston and crank mechanism.